Sunn Concert lead PA

[gbono]

I have a 1973 Sunn Concert Lead that was dropped and then butchered by someone trying to repair the poor thing. I have everything back together but have a problem with the power amp - actually Q3 (see PA schematic).

After power up with no signal input or load - R304/308 reaches about 200F in 30 seconds and Q3 isn't far behind in temperature. All other output devices and drivers are not getting hot. Q3 has about 46V on the collector and there is about 5 volts on the base and 4.3V on the emitter (yup it's an emitter follower). I've checked all the traces, power supply rails and components in the circuit and can't find a reason for the overheating (oddly the timmer pot R310 has no effect but tests out fine.)

Any ideas?
ConcertLPA.pdf

[gbono]

Okay I measured the voltages on the output transistors and I'm seeing an issue with the bias on all devices.

Q2 Q3 Q4 Q5
VCE 4.6V 46V 46V 3.9V
VBE 3.4 4.8V 3.9V 4.8V
VE 4.2

Okay the voltages above are referenced from either the "-" terminal of C2 or the "-" terminal of C1.

The values for VCE should read around 23V for Q2 and Q5 but they're not. The base bias is really wrong - should be 1/2 of VCC. I didn't measure IE (didn't have time). I'm almost 100% certain that the transistors are fine - they're new. I did notice that the TO-3 sockets have loose pins (low insertion force) for the emitter and base but the devices are making contact.

Check the grounding scheme and found no issues - though it looks like this design lands chassis ground at the output connector. All the bias resistors check out good and I replaced a few for good measure.

Frustrating since this is a relatively simple emitter follower circuit - start looking at the feedback loop and components therein??

[gbono]

This is my online journal entry for 8/16 just in case someone is interested ;-)

I have removed all the output devices and R304 and R303 are still heating up. I did find a short that the previous "tech" had snuck in - connecting node 2 to 4. The VC (collector voltage) on all the output sockets is now correct at 1/2VCC = 48/2= 24V. I replaced all the bias resistors on Q2-Q5 for good measure (some had drifted) but that's not the fix.

What else should I look for to see if the bias is balanced? Since the secondaries on the interstage transformer are floating - I'm assuming that, unless there is a short to ground on S1,T1 or T1,S2, they don't figure into this. Yes it did run a continuity check on the above secondaries and they don't appear to be shorted to ground. What else he asks??